Historical Articles

A Flexible Plated Circuit

E.R. Bowerman
and R.F. Walton

This paper was presented at the
National Conference on Airborne Electronics, Institute of Radio Engineers,
Dayton, OH, May 1953.

ABSTRACT
A cloth backed plated circuit is made by a transfer process by which a
strippable copper electrodeposit and a thin polyvinyl resist are transferred
to a cloth
adhesive tape from a stainless steel base. The stainless steel is reusable
and its surface may be patterned to identify the wiring on the unit
being produced.
The transfer method eliminates the need for providing, between the
wires, of conductive bridges which have to be removed later. Design
changes
require only
a change in the silk screen used to apply the resist to the stainless
steel.

INTRODUCTION
The flexible plated circuit appears to fit nicely into the general methodology
of fabrication of electronic equipment. Some of the current forms
of printed circuitry are rigid and this is, in fact, made use of in supporting
small
components. On the other hand, larger equipment is going to be
chassis
bound for some time
to come. Here, experience has shown that a state of fluctuation
exists in individual terminal to terminal dimensions and elevations. Individual
wires
easily take
this variation-into account as can pig-tailed components. If the
circuit
is complex, it is not unusual for a number of small components
to be sub assembled on a terminal
board where dip soldering can be used to advantage.

THE PROCESSThe production of a flexible plated circuit, simply stated, consists
in silk screening a stop-off (i. e., an insulating lacquer) onto
a stainless steel
surface, copper plating the non stopped-off area, and stripping
the plating and stop-off
from the stainless surface by an adhesive tape. This is illustrated
in the flow diagram (Fig. 1) which will serve as an outline of
the description
to
follow.

Stainless Steel PreparationType 316 or 347 stainless steel sheet, free from blemishes, flaws
and fissures, is chosen because of its resistance to chemical
attack in
the copper plating
solution and because it can be easily maintained in the passive
condition necessary for stripping the copper plating. The surface
is given a
mirror finish by polishing
and buffing and care is exercised in handling to avoid scratches.
Surface roughness has a tendency to cause adhesion of copper
plating to the
steel, possibly by
mechanical bonding since the difficulty is noticed even with
properly passivated steel. Individual scratches will be faithfully
reproduced
in the plating
and, if sharp, will have a tendency to cause local mechanical
weakness in the copper
probably as the result of a notch effect. After polishing and
buffing, the surface is cleaned and passivated by anodic treatment
in a conventional
hot
alkaline
steel electrocleaning solution.

Silk ScreeningA reversed silk screen is prepared by conventional graphic
arts procedure using a fine mesh wire cloth (140 or 160 US
Std) and
a film which is
resistant to the
stop-off lacquer and thinner. The reversed screen is necessary
because of the subsequent reversal when the circuit is stripped
from the stainless
steel.

The stop-off lacquer is a vinyl
copolymer whose viscosity is adjusted to approximately 200 poises by means
of suitable
solvents.
Care is
used in the
screening to avoid
holes or discontinuities in the lacquer film other than
those of design. The evaporation rate of the solvent is adjusted
in a compromise between
rapid drying
of the screened film and slow drying on the screen.

Fig. 1. Flow diagram.

Fig. 2. Conventional chassis wiring.

Fig. 3. Chassis with plated circuit.

Electroforming
Copper WiringAfter the stop-off is dry, the wiring is formed by electroplating
between 1 and 3 mils of copper onto the non stopped-off
stainless steel in
an acid copper sulfate
plating solution. A practical lower thickness limit exists
in that deposits thinner than 0.7 mils have a tendency
to tear when
being stripped
from
the stainless
steel. This is postulated to be due to a lower strength
within the copper than in the weak bond between the copper
and the
stainless steel.
An
upper thickness
limit is less easily defined but as the thickness increases
the flexibility of the copper decreases and the deposit
tends to
spread along the surface
over the
stop-off. If the wiring is closely spaced, bridging may
occur between wires due to the spreading deposit. For
the circuitry
investigated,
loss in flexibility
gave an upper limit at about 20 mils.

The plating solution
used for the investigative work contains 250 g/l CuSO4 · 5H2O and 75 g/l
H2SO (sp. gr.
1.83). The
solution was filtered
through
activated
carbon before using and commercial electrolytic cathode
copper was used as anode. The stopped-off stainless
steel is connected
as cathode
before
immersion
in the
plating solution so that plating starts immediately
on the passive surface. The cathode is agitated during plating
and
a cathode
current density
of 100 amperes
per square foot is used. In 18 minutes a 1.5 mil deposit
is obtained which is suitable for 1/16 inch wide wires
in receivers
having
series string heater
connections
with 150 milliampere currents. After plating the cathode
is water rinsed to remove the plating solution and
dried.

Stripping the CircuitAn industrial cloth backed adhesive tape is pressed
against the copper plating and stop-off with sufficient
pressure
and time
to obtain
good contact to
the adhesive. The tape and adhered circuit is then
stripped from the steel sheet
with a rapid, continuous, even pull. The stop-off
also transfers and serves to cover the sticky areas of the
tape not covered by the
copper. The particular
tape used was compounded using a thermosetting adhesive
which could be cured at 250° F before use or
allowed to cure in equipment. After stripping, the
tape is trimmed and the required holes punched
out. The stainless steel
may be
reused as long as the surface is maintained smooth,
clean and passive.

Installation of the Plated CircuitThe flexible circuit is forced over the fixed component
and tie point lugs before the small components
are installed. The flexibility
of
circuit harness
speeds
up the installation by the give and take possible
in cloth fabrics. Some attention to the design
of the
lugs will
pay
off in easing
the assembly
and reducing the
necessity for hole punching to accommodate lugs.
On the other hand, if highly detailed work is not
being
produced
it is
also possible to
use
a loose woven
fabric for the adhesive tape backing which will
permit the lugs to pierce it easily.

Coding of the WiringSince the electroforming of the copper will faithfully
reproduce the surface finish of the stainless
steel, it is possible
to transfer certain
types of
designs to the copper surface beyond the obvious
capabilities of outline identification
techniques. Localized texturing of the stainless
steel as, for example, by shot blasting, etching
or engraving,
will
give a
visual coding to
the plating.
Care
must be used to employ only rounded and relieved
designs to permit parting after plating. In addition,
the deposit
may
have to be
made a little
thicker to obtain
good strength for the parting operation.

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